This invention relates generally to turbine components and more particularly to a combustor liner.
Conventional gas turbine combustors use diffusion (i.e., non-premixed) combustion in which fuel and air enter the combustion chamber separately. The process of mixing and burning produces flame temperatures that can exceed 3900 degrees F. Since conventional combustors and/or transition pieces having liners are generally capable of withstanding for about ten thousand hours (10,000 hrs.), a maximum temperature on the order of about 1500 degrees F., steps to protect the combustor and/or transition piece must be taken. This has typically been done by film-cooling which involves introducing relatively cool compressor air into a plenum formed by the compressor discharge case surrounding the outside of the combustor. In this prior arrangement, the air from the plenum passes through louvers in the combustor liner and then passes as a film over the inner surface of the liner, thereby maintaining combustor liner temperature at an acceptable level.
Because diatomic nitrogen rapidly disassociates at temperatures exceeding about 3000° F. (about 1650° C.) and reacts readily with oxygen at such temperatures, the high temperatures of diffusion combustion result in relatively high NOx emissions. One approach to reducing NOx emissions has been to premix the maximum possible amount of compressor air with fuel. The resulting lean premixed combustion produces cooler flame temperatures and thus lower NOx emissions. Although lean premixed combustion is cooler than diffusion combustion, the flame temperature is still too hot for prior conventional combustor components to withstand without some type of active cooling.
Furthermore, because the advanced combustors premix the maximum possible amount of air with the fuel for NOx reduction, little or no cooling air is available, making film-cooling of the combustor liner and transition piece impractical. Nevertheless, combustor liners require cooling to maintain material temperatures below limits. In dry low NOx (DLN) emission systems, this cooling can only be supplied as cold side convection. Such cooling must be performed within the acceptable limits of thermal gradients and pressure loss. Thus, means such as thermal barrier coatings in conjunction with “backside” cooling have been utilized to protect the combustor liner and transition piece from destruction by such high heat. Backside cooling involves passing the compressor air over the outer surface of the combustor liner and transition piece prior to premixing the air with the fuel.
There are currently three forms of prior art for the convective cooling of combustor chambers. First, a series of longitudinal or axially spaced horizontal turbulators, which appear as straight lines across the surface of the liner, are used in practice to disrupt the thermal boundary layer and provide enhanced heat transfer for cooling. These turbulators are either machined in the metal surface, or applied as tack-welded strips of material to the metal. Second, convective cooling is provided by a series of impingement jets supplied by the external combustor chamber cooling flow sleeve. Typically, it is not possible to provide such impingement cooling over the entire extent of the chamber, and so some mixture of impingement and surface turbulators is employed. Third, an array of surface indentations, also known as dimples or hemispherical concavities, is made in the liner surface to create flow vortices that act to enhance heat transfer. The various known techniques enhance heat transfer but with varying effects on thermal gradients and pressure losses.